Not backing up

 All hard drives will die. Even sold state drives will fail. All data must be backed up. Even new computers. Data should live in 3 locations, 2 local, 1 remote. 

Running Norton or McAfee after being infected

 If you have security issues, you need better protection than the popular consumer solutions. 

Not running system updates

 All system updates must be installed. This includes Macs. Any security fix is not optional. 

Running obsolete operating systems or programs

 A program may have been a great solution in years past. That doesn’t mean that it’s a great solution today. Security is much more important than in the past. 

Ignoring serious errors, like disk warnings

Errors are not to be ignored. Disks will sometimes warn before failing. If you don’t understand the error, find someone who does. 

Going to the same sketchy websites over and over

If the website makes your security software unhappy, stay away from it. If you get problems from a site, don’t return to it.